A Physics Puzzler

February 23rd, 2007

I was eating dinner and sharing a couple bottles of wine with my friend Mike Fitzmorris last night, and he posed me this two-part question to which neither of us are quite sure about the answer:

Part one: Imagine an iron bar in free-space (no gravity, perfect vacuum, perfect darkness). If you impart a force such that it spins about the short axis, does it ever stop spinning?

It seems that with nothing around to apply a counter-force to the bar, it should spin forever, cosmological and bizarre quantum effects aside.

Part two: Imagine the same iron bar under the same conditions, but magnetized. If you impart a force such that it spins about the short axis, does it ever stop spinning?

The answer to this part seems to be that it should stop spinning eventually, but I can’t explain exactly why. The spinning magnetized iron bar would generate a fluctuating magnetic field, which means that it would be emitting some kind of remotely detectable signal, which, by definition, means that energy is being lost and therefore the bar should eventually stop spinning…

But exactly what is applying the counter-force to cause the bar to stop spinning? The bar is in a perfect free-space, so the electromagnetic waves will never interact with anything…The best explanation I could come up with is that the bar’s self-magnetic field is “catching up” with its emitted magnetic signature, and this interaction induces a counter-current in the metal which would cause an opposing magnetic field that would result in a net braking effect. I’m not very happy with this explanation, however.

Anybody know the correct answer to this question, or have a good explanation for why it might be so? There must be a simple answer and I’m just not seeing it.

[some edits and an addendum here to clarify this post]

A lot of interesting views in the comment area, thanks everyone for the thoughts!

Let me rephrase the thought experiment, because I think my poor phrasing has lead to many people to point toward flaws in the problem statement rather than the problem itself.

Suppose we think now of the spinning magnet in free space as a flywheel, and I couple energy out of the flywheel via a large coil. The changing magnetic field of this flywheel would cause electrons to move in the coil, and clearly since I am coupling power out of the flywheel via the coil’s capture of the changing magnetic field, the flywheel must slow down, otherwise energy is not conserved. In other words, the flywheel’s kinetic energy is converted into electrical energy by this mechanism.

If I remove this coupling coil, no energy is being directly removed from the system, so you might think, okay, the magnetic flywheel should not spin down ever since there are no mechanical frictional effects.

However, now imagine that I create an enormous superconducting coil that spans a diameter of 10 light seconds away from the flywheel. As I start spinning the flywheel, the changing magnetic field flows away at the speed of light. If the argument is that no energy is lost into this magnetic field, then the flywheel will not slow down at all. However, 10 seconds later, this magnetic field hits the coil, and all this energy is captured in the coil…but where did this energy come from?

Clearly, at time = 10 seconds, the flywheel can’t possibly begin instantaneously spinning down; the information about the presence of the coil would have to propagate its way back to the flywheel first, otherwise we have created a mechanism for transferring information at a rate faster than the speed of light. So for the “flywheel does not slow down” hypothesis to be valid, for a net 10 seconds, the system mysteriously has “extra” energy–I took energy out via the coil, yet the flywheel maintained its original rate of spinning and all its kinetic energy for the 10 seconds that it took for the information about the existence of the coil to make it back to the magnetic flywheel.

Therefore, in the absence of any device to couple energy out of the system (other than radiation to free space), the flywheel must slow down with time, otherwise I can spontaneously create energy. Therefore, there must be a completely local mechanism within the flywheel that causes the energy to be debited at the instant that it radiates from the flywheel. This is true whether the bar spins near the speed of light, or if it spins very slowly as any changing magnetic field can compel an electron to move and therefore transfer energy to another part of the system.

It is this local energy-debiting mechanism that I am trying to grasp. Any explanation also has to work if I replace the iron bar with a non-conductive magnet, like a ceramic or organic magnet. That’s a flaw in my counter-eddy current explanation that I proposed above.

Here is a quick sketch that helps illustrate the question.

Posted in Ponderings | 41 Comments »

Where Have All the Innovators Gone?

February 13th, 2007

People have often asked me, now that I have some perspective on China, what I think will happen to the US. Can we compete? Will we continue to lead? I’m quite bullish about the US in general, but I had an interesting reality check tonight. I’m at ISSCC 2007 right now (where I and my former colleagues at Luxtera had the honor of receiving an “outstanding paper” award for work presented at last years’ conference), and I was chatting with UCSD high speed integrated circuits professor Jim Buckwalter about the nature of the graduate student applications he has received.

The statistics were astonishing. Of the thousands of applicants, only 80 were from the US. To put this in perspective, he had more applicants with the surname “Lee” alone than he had domestic applicants. And UCSD engineering is no slouch; according to the rankings they are #11 in engineering overall. Even more interesting is that apparently Korean students studying in the US get Korean-government sponsored fellowships–clearly that gives them an edge when considering who to take into your graduate program.

The enormous disparity in domestic applicants to higher education in crucial fields such as high speed circuit design is a bit disturbing. With numbers like these, it is inevitable that the US will lose its edge in technology. I guess it wouldn’t be as bad if these foreign students actually stayed in the US and started companies, but my experience in China has shown that just about every company I talked to had US-educated management from schools like Berkeley and Stanford.

Now, a protectionist mode of thought would suggest that we should put quotas on the number of foreign people we admit to our universities. That doesn’t work because US citizens don’t want to go to graduate school in electrical engineering, as evidenced by the paltry showing of domestic applicants, and forcing them in doesn’t make us more competitive in the global sense.

As the son of Chinese immigrants born and raised in the cornfields of Michigan, clearly I’m disposed to argue that we should try harder to woo these brilliant foreign minds to graduate and set up shop here in the US. Back when my parents came, staying in the US was an easy decision, because China was not a land of opportunity. But in this new global economy, the US no longer has the monopoly on opportunity. That’s the big paradigm shift here that I think we aren’t internalizing. We are no longer “the land of opportunity”– we’re now just one of the better places to find an opportunity.

When you don’t have a monopoly, it means there is competition. We need to compete to retain foreign talent, but instead, we hassle them away. I just wrote a green card recommendation for a brilliant photonic circuit designer. It seems weird that he has received such scrutiny and is going through such detailed background checks when anyone who lives in a border town like San Diego knows there is another easier way for immigrants to sneak into the country and make a living–and I have a feeling the guys sneaking in don’t have PhDs in electrical engineering. And it really bugs me that a brilliant Iranian circuit designer friend of mine just got interrogated by the FBI out of the blue, but presumably motivated because of current events in the world. He’s not a terrorist, and he doesn’t make nukes, despite his Farsi-sounding name. He is a core technical contributor in a US electronics company whose work has been critically peer-recognized as innovative and valuable. We should be rolling out the red carpet for these innovators, and not making them feel like aliens.

While I understand the motivations of many of our immigration policies, it is becoming clear to me that in practice, something is broken here, and the loser will be the US. The beauty of a melting pot is that we have the opportunity to incorporate the best and brightest minds into our culture; instead we skim the cream and throw it away, simply because they are the easiest and most cooperative targets. The system is hassling the people who are educated, and rewards those who are not. This is because the educated ones know the rules and are held to a high standard, and those who don’t know the rules often are not well educated so they have well-meaning public advocates who try to represent and defend their interests.

Of course, these privileged foreigners don’t need public advocates, and they don’t make a fuss, so their problems rarely garner the attention of the public eye. They are resourceful, self-sufficient, and they have other options–if the US gives them the run around, they can always take their good ideas and start a company back at home.

Posted in Made in China, Ponderings | 39 Comments »

C64 vs. Xbox360

February 9th, 2007

Nate Lawson (co-designer of the Blu-Ray content protection layer) had a nice presentation at RSA2007 comparing the content protection schemes used on the Commodore64 vs. the Xbox360. His slides are an interesting and fun read…kind of brings out a certain nostalgia, but then you realize that things aren’t all that different today.

Posted in Hacking | 5 Comments »

Seeing Through Circuit Boards

February 7th, 2007

I’m resolving some manufacturing challenges in the new chumby design and had the opportunity to use an X-ray board inspection tool. This tool is what you want to use if, for example, you wanted to reverse engineer a circuit board with buried or hidden traces. It also lets you see through the packages and inspect the quality of the wirebonds. This particular system has a feature that let you rotate the board as you inspected it so you can look at it at an angle–which is actually really cool to see live, it’s sort of like flying through the circuit board. I wish I had a video of it but I didn’t bring my camera into the shop today.

Anyways, I thought those who have never seen this capability before would find the following pictures interesting and perhaps thought provoking.

Nothing hides from the X-ray eye!

Posted in chumby, Hacking | 12 Comments »

What Nerds do on a Friday Night

February 4th, 2007

What do you get when you mix mexican food, a healthy portion of margaritas, nerds, and a laser cutter? You wake up with a tattoo on your cell phone. Pana documents the results in her blog.

Posted in Social | 12 Comments »

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